1. Field of the Invention
This invention relates to the chemical treatment of aqueous solutions such as waste water to remove dissolved cyanides.
2. Description of the Prior Art
It is known that concentrations of a few parts per million of soluble cyanides (e.g. sodium cyanide) are toxic to the microflora and microfauna which comprise the food-chain of higher forms of aquatic life such as fish, waterfoul, and eventually man. For this reason the United States Environmental Protection Agency (E.P.A.) has enacted strict laws to regulate the amount of soluble cyanides which may be discharged from any source into natural waters.
For certain industrial operations, such as the extraction of gold and silver from their ores, soluble cyanide compounds (e.g. sodium cyanide, potassium cyanide) are essential reagents used in the extraction process. In earlier years, mining and other industrial companies traditionally discharged their waste waters, containing sometimes as much as 50-100 parts per million (ppm) of soluble cyanide, into streams or rivers. It was assumed that the relatively small concentrations of soluble cyanides in the waste waters would be greatly diluted, dissipated and inactivated by the natural stream of river waters.
Numerous studies by ecologists, limnologists, and environmental scientists have demonstrated that concentrations of free, chemically uncomplexed, cyanide ion (CN.sup.-) as low as 1 ppm are toxic to microflora and microfauna comprising the food-chain of fish and other animals.
For operations such as mining, electroplating, and similar industries which produce large volumes of waste waters containing soluble cyanides in concentrations in the range 1-50 ppm or more, the E.P.A. has enacted regulations which prohibit the discharge of waste waters that contain more than 0.02 ppm (i.e., 1 part in 50 l million) to such cyanides.
Compliance with this extremely low concentration of cyanide in industrial waste waters which are discharged to the environment has presented enormous problems to the industries that must meet such standards.
Aeration of acidified solutions containing free cyanide ion results in only limited removal of cyanide as gaseous hydrogen cyanide. This method is not effective for the removal of complex cyanides with metals such as zinc, nickel, copper, cobalt, iron, etc. In acidified and aerated solutions, these complexes gradually decompose and free cyanide ion increases exponentially. What is urgently needed, therefore, is a process that will remove free or complex cyanide ions from solution as a stable precipitate and which will leave no detectable soluble cyanide species in solution. Such a process should also be rapid, economically feasible and industrially practicable. The present invention provides a process which attains these criteria.
The use of ferrous sulfate to precipitate soluble cyanide ion as the very insoluble compound, Prussian blue (ferric ferrocyanide) has been known for many years. This process has been shown to be effective in reducing the concentration of free or complex cyanide ion from relatively high initial concentrations (e.g. 100, 500, 1000 ppm) to very low concentrations of total cyanide in the supernatant solution. Unfortunately, the supernatant solution over the Prussian blue precipitate is found always to contain approximately 0.5-3.0 ppm cyanide ion, depending upon the conditions of treatment of the solution with ferrous ion. This concentration of cyanide is well above that which is allowed by current E.P.A. regulations in waste waters.
Treatment of the solution resulting from Prussian blue precipitation by passage through suitable ion exchange resins has met with only partial or limited success. Although some ion exchange resins can reduce the total cyanide concentration from the initial 0.5-3.0 ppm to less than 0.02 ppm, industrial practice has demonstrated that the efficiency of removal of total cyanide ion rapidly deteriorates as the active adsorption sites on the ion exchange resin become covered. The result is that while some ion exchange resins initially have the ability to reduce the total cyanide concentration in the supernatant solution from Prussian blue precipitation from approximately 0.5-3.0 ppm to less than 0.02 ppm, it is found that the efficiency of these resins falls off rapidly, and the legally permitted upper limit of 0.02 ppm cyanide is soon exceeded.
A further disadvantage of ion exchange resins is their need for regeneration, to desorb the adsorbed complex ions resulting from the treatment of the original solution with ferrous ion. While ferrocyanide and ferricyanide ions can usually be stripped from the resins fairly easily by the passage of suitable concentrated eluting solutions, other complex cyanides (e.g. cuprocyanide, cupricyanide, cobalticyanide) present major problems of removal, requiring extremely long and industrially impractical contact times with the eluting solutions. In many cases it has been found that not all of the adsorbed complex cyanide ions can be removed by eluting solutions. This, of course, progressively reduces the adsorption sites and consequently the efficiency of the regenerated resin.
There exists an urgent requirement, therefore, for a process in which the supernatant solution resulting from Prussian blue precipitation is treated chemically (not physicochemically, as in ion exchange) to reduce the cyanide concentration dependably to less than 0.02 ppm, or to "undetectable" levels.
One object of this invention is to provide a process by the use of which an aqueous solution containing free or complex cyanide ions is treated chemically to reduce the concentration of such ions to analytically undetectable levels.
Another object of this invention is to provide a process by the application of which an aqueous solution containing cyanide ions which are either uncomplexed with metals or complexed with metal ions as coordination compounds are treated to reduce the concentration of such uncomplexed or complexed cyanide ions to analytically undetectable levels in an industrially practicable and economic manner.
A further object of this invention is to provide a process by the use of which complexed or uncomplexed cyanide ions are treated to reduce the concentration of such cyanide ions to analytically undetectable levels and to produce a solution which is not toxic to aquatic or terrestrial organisms.
A further object is to provide a process the application of which is environmentally acceptable.